This invention relates to multi-antenna receivers.
Future wireless communications systems promise to offer a variety of multimedia services. To fulfill this promise, high data rates need to be reliably transmitted over wireless channels. The main impairments of wireless communication channels are time varying fading due to multipath propagation, and time dispersion. The multipath fading problem can be solved through antenna diversity, which reduces the effects of multipath fading by combining signals from spatially separated antennas. The time dispersion problem can be solved by equalization, such as linear, decision feedback, and maximum likelihood sequence estimation (MLSE).
It has been a standard practice to use multiple antennas at the receiver with some sort of combining of the received signals, e.g., maximal ratio combining. However, it is hard to efficiently use receive antenna diversity at remote units, e.g., cellular phones, since they typically need to be relatively simple, small, and inexpensive. Therefore, receive antenna diversity and array signal processing with multiple antennas have been almost exclusively used (or proposed) for use at the base station, resulting in an asymmetric improvement of the reception quality only in the uplink.
Recently, there have been a number of proposals that use multiple antennas at the transmitter with the appropriate signal processing to jointly combat the above wireless channel impairments and provide antenna diversity for the downlink while placing most of the diversity burden on the base station. Substantial benefits can be achieved by using channel codes that are specifically designed to take into account multiple transmit antennas. The first bandwidth efficient transmit diversity scheme was proposed by Wittneben and it included the transmit diversity scheme of delay diversity as a special case. See N. Seshadri and J. H. Winters, “Two Schemes for Improving the Performance of Frequency-Division Duplex (FDD) Transmission Systems Using Transmitter Antenna Diversity,” International Journal of Wireless Information Networks, vol. 1, pp. 49–60, January 1994. In V. Tarokh, N. Seshadri, and A. R. Calderbank, “Space-Time Codes for High Data Rate Wireless Communications: Performance Criterion and Code Construction,” IEEE Trans. Inform. Theory, pp. 744–765, March 1998, space-time trellis codes are introduced, and a general theory for design of combined trellis coding and modulation for transmit diversity is proposed. An input symbol to the space-time encoder is mapped into N modulation symbols, and the N symbols are transmitted simultaneously from N transmit antennas, respectively. These codes were shown to achieve the maximal possible diversity benefit for a given number of transmit antennas, modulation constellation size, and transmission rate. Another approach for space-time coding, space-time block codes, was introduced by S. Alamouti, in “Space Block Coding: A Simple Transmitter Diversity Technique for Wireless Communications,” IEEE Journal on Selec. Areas. Commun., vol. 16 pp. 1451–1458, October 1998 and later generalized by V. Tarokh, H. Jafarkhani, and A. R. Calderbank, in “Space Time block Codes From Orthogonal Designs,” IEEE Trans. Inform. Theory, vol. 45, pp. 1456–1467, July 1999.
Space-time codes have been recently adopted in third generation cellular standard (e.g. CDMA-2000 and W-CDMA). The performance analysis of the space-time codes in the above-mentioned articles was done assuming a flat fading channel. Analysis shows that the design criteria of space-time trellis codes is still optimum when used over a frequency selective channel, assuming that the receiver performs the optimum matched filtering for that channel. In addition, although the space-time coding modem described in A. F. Naguib, V. Tarokh, N. Seshadri and A. R. Calderbank, “A Space-Time Coding Based Modem for High Data Rate Wireless Communications,” IEEE Journal on Selec. Areas Commun., vol. 16, pp. 1459–1478, October 1998 was designed assuming a flat fading channel, it performed remarkably well when used over channels with delay spreads that are relatively small as compared to the symbol period Ts. However, when the delay spread is large relative to the symbol period, e.g., >Ts/4, there was a severe performance degradation.